This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-156484, filed May 26, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a semiconductor device such as a transistor and a method of manufacturing the semiconductor device, and more specifically, a SiGe bipolar transistor having a high current gain and a manufacturing method thereof.
An npn-junction conductive type transistor having a high current gain is usually manufactured by the following method. First, a p-conductive type SiGe film 22 and an n-conductive type Si film 23 are sequentially stacked on an n-conductive type Si substrate 21 by chemical vapor deposition (Step S21), as shown in FIGS. 1A, 1B and 2. Then, phosphorous (P) ions supplied from a P ion source 24 are implanted. The resultant structure is subjected to a high-temperature annealing treatment to activate the P ions implanted. As a result, a heavily phosphorus-doped layer 25 is obtained (Step S22). Subsequently, as shown in FIGS. 1C-1E and 2, a part of the heavily phosphorus-doped layer 25 and the n-conductive type Si film 23 are removed by a milling method or reactive ion etching to expose a base surface (Step S23). Thereafter, the resultant structure is subjected to a mesa-etching step to form a mesa etching portion 27 (Step S24). After that, a collector electrode 28, a base electrode 29, and an emitter electrode 30 are independently connected to appropriate portions (Step S25).
In a conventional SiGe transistor, defects easily occur in a p-conductive type SiGe film employed as a base layer. Because of the defects, the lifetime of carriers within the p-conductive type SiGe film tends to be shorter than that of a Si film. Consequently, the switching speed of the SiGe transistor becomes faster than that of a Si transistor. Therefore, the SiGe transistor can serve as a high-speed transistor. However, the p-conductive type SiGe film is low in mobility due to the short lifetime. For this reason, the current gain of the conventional SiGe transistor tends to be lower than that of the Si transistor.
The present invention was attained to overcome the aforementioned problems. An object of the present invention is to provide a semiconductor device such as a SiGe bipolar transistor exhibiting a high current gain and a method of manufacturing the same.
According to the present invention, there is provided a semiconductor device comprising:
a Si substrate of a first conductive type;
a first Si film of the first conductive type formed on the Si substrate;
a SiGe film of a second conductive type formed on the first Si film;
a second Si film of the second conductive type formed on the SiGe film;
a third Si film of the first conductive type formed on the second Si film;
a first electrode formed by removing a part of the third Si film or changing the conductive type of the part of the third Si film of the first conductive type to the second conductive type, and joining a metal terminal to a part of the second Si film exposed by removing the part of the third Si film or a part of the third Si film changed in conductive type;
a second electrode formed by joining a metal terminal to the third Si film; and
a third electrode formed by joining a metal terminal to a back surface of the Si substrate.
In the semiconductor device of the present invention, the base is formed of two layers: one is a SiGe film of a second conductive type (p-conductive type) within which the lifetime of carriers tends to be short and their mobility tends to be slow, and the other is a Si film of a second conductive type (p-conductive type) within which the lifetime of carriers is long and their mobility is fast. Due to this, the current gain can be greatly improved. Furthermore, a thickness ratio of the p-conductive type SiGe film and the p-conductive type Si film can be varied in the present invention. It is therefore possible to change the current gain variously. It should be noted that the thickness ratio of both films is not zero. This is because if the thickness ratio is zero, the entire base layer is made of the p-conductive type Si film. This case is not preferable since the switching characteristics deteriorate.
Note that the SiGe film and the second Si film constituting the base are preferably equal in thickness. The total thickness of the SiGe film and the second Si film constituting the base preferably falls within the range of 200-400 nm. The lowermost value of the total thickness of the base is set at 200 nm. This is because if the total thickness of the base is lower than 200 nm, the voltage rating of the transistor is degraded. On the other hand, the uppermost value is set at 400 nm. This is because if the total thickness exceeds 400 nm, the current gain decreases.
The base is not limited to the aforementioned two-layer combination, that is, the p-conductive type SiGe film and p-conductive type Si film. The base may be formed by stacking three or more p-conductive type layers. For example, the three layers base may be formed of the p-conductive type Si film, the p-conductive type SiGe film having X% Ge concentration, and the p-conductive type SiGe film having y% Ge concentration. Alternatively, the base may be a multi-layered structure such as an eleven-layered structure.
Furthermore, it is preferable that the Ge concentration of the p-conductive type SiGe film at a side near an emitter be set at 0% (that is p-conductive type Si film) and increased toward the side near a collector. Incidentally, the upper limit of the Ge concentration of the P-type SiGe film is desirably 20 atomic %. This is because if the Ge concentration is further high, dislocations are formed in the p-conductive type SiGe film. The dislocation decreases the quality of the film and thus degrades transistor characteristics.
According to the present invention, there is provided a method of manufacturing a semiconductor device comprising:
(a) applying a first raw material gas to a surface of a Si substrate of n-conductive type while the substrate is heated in a vacuum chamber to form a first Si film of n-conductive type on the substrate;
(b) applying a second raw material gas to a surface of the first n-conductive type Si film under heating to form a SiGe film of p-conductive type on the first Si film;
(c) applying a third raw material gas to a surface of the SiGe film of p-conductive type under heating to form a second Si film of p-conductive type on the SiGe film;
(d) applying a fourth raw material gas to a surface of the second Si film under heating to form a third Si film of n-conductive type on the second Si film of p-conductive type;
(e) implanting a high concentration of phosphorus ion to a surface layer portion of the third Si film of n-conductive type and annealing the surface layer portion to activate the phosphorus ion implanted;
(f) removing a part of the third Si film due to the second Si film or changing the conductive type of a part of third Si film to another conductive type; and
(g) joining a metal terminal to a part of the second Si film exposed by removing the third Si film or the part of the third Si film of another conductive type to form a first electrode, joining a metal terminal to the third Si film to form a second electrode, and joining a metal terminal on a back surface of the Si substrate of n-conductive type to form a third electrode.
When a power transistor having a high current gain is formed as a semiconductor device, it is desirable to employ an n-conductive type Si substrate having a low resistivity as low as 0.1 xcexa9xc2x7cm or less. As a film-formation means, a thermal CVD apparatus using chemical vapor deposition is used.
In the step (a), the first raw material gas contains phosphine of 0.1 ppm or less and the rest being disilane Si2H6. By using the first raw material gas, it is desirable to form a P-doped n-conductive type Si film of 20-50 xcexcm thick containing P in an amount of 1xc3x971015 atom/cm3.
In the step (b), the second raw material gas contains 1-25 atomic % of germane GeH4, 1 to 1xc3x97103 ppm of boron, and the rest being disilane Si2H6. By using the second raw material gas, it is desirable to form a B-doped p-conductive type SiGe film of 0-400 nm thick containing B in an amount of 1xc3x971016-5xc3x971017 atom/cm3.
In the step (c), the third raw material gas contains 1-1000 ppm of boron and the rest being disilane Si2H6. By using the third raw material gas, it is desirable to form a B-doped Si film of 0-400 nm thick containing B in an amount of 1xc3x971016-5xc3x971017 atom/cm3.
It is desirable that the total thickness of the B-doped p-conductive type SiGe film formed in the step (b) and the B-doped Si film formed in the step (c) should fall within the range of 200-400 nm.
In the step (d), the fourth raw material gas contains phosphine of 1xc3x97102 to 1xc3x97104 ppm and the rest being disilane Si2H6. By using the fourth raw material gas, it is desirable to form a P-doped n-conductive type Si film of 100-600 nm thick containing P in an amount of 1-8xc3x971018 atom/cm3.
In the step (e), P is implanted in an amount of 1xc3x971014 to 1xc3x971016 atom/cm2 at an acceleration energy of 10-50 keV. The annealing is desirably performed at a temperature from 700 to 1000xc2x0 C. for 3-60 minutes.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.